The present invention relates to a magnetic screw used as a feed device or the like in machine tools and other machines and instruments.
Magnetic screws are used as feed devices or the like in machine tools, machines and instruments installed in clean rooms and other various machines and instruments. A magnetic screw comprises a screw shaft of magnetic material having thread crests formed on its outer peripheral surface, and a magnetic nut fitted on the outer periphery of the screw shaft with a minute gap defined therebetween. And when the screw shaft is turned around its axis forward and backward, the magnetic nut is axially reciprocated along the screw shaft by the action of the magnetic coupling between the screw shaft and the magnetic nut.
A magnetic screw, unlike a ball screw, does not require the screw shaft and the magnetic nut to be in direct contact with each other, so that it can be used without lubrication and even if it is used for a long time there is no error occurring due to thermal expansion: thus, since it can be used with low noise, low vibration and low dust generation and is superior in durability, its applications are being broadened.
Magnetic screws are classified into two types, one in which the magnetic nut is constituted by incorporating a number of permanent magnets, as described in Japanese Patent Kokai Hei 1-176850, and the other in which the magnetic nut is constituted by magnetizing the inner peripheral surface of a magnetic sleeve, as described in Japanese Patent Kokai Hei 8-17625.
That is, the former magnetic screw comprises a screw shaft of magnetic material having thread crests formed on its outer periphery, and a magnetic nut having a housing internally provided with a number of magnets spirally continuously disposed through a holding member in such a manner that the magnets correspond with the thread crests of the screw shaft.
The latter magnetic screw, as shown in FIG. 9, comprises a screw shaft 30 of magnetic material having thread crests 31 formed on its outer periphery, and a magnetic nut 32 having a housing 33 internally provided with a cylindrical yoke 34 and magnet bodies 35, the inner peripheral surfaces of said magnet bodies 35 being spirally magnetized so that they correspond with the thread crests 31 of the screw shaft 30.
Since the former magnetic screw requires a number of permanent magnets to be spirally arranged internally of the housing of the magnetic nut, there are disadvantages that it is structurally very complicated, that the operation of mounting these permanent magnets on the inner periphery of the housing is very troublesome, and that the magnetic screw lacks in mass productivity, leading to a large increase in manufacturing costs. Furthermore, there are other disadvantages that it is difficult to arrange a number of permanent magnets spirally continuously without a gap and that the axial thrust force in the screw shaft cannot be increased.
Since the latter magnetic screw employs the construction in which the inner peripheral surfaces of the magnet bodies 35 are spirally magnetized to correspond with the thread crests 31 of the screw shaft 30, it is simpler in construction, easier to produce, less expensive and has higher mass productivity than in the former. Further, since the inner peripheral surfaces of the magnets bodies 35 are spirally continuously magnetized, there is another advantage that the axial thrust force can be relatively readily obtained as compared with the former case. In the case of this latter magnetic screw, however, since N- and S-poles are alternately spirally arranged on the inner peripheral surfaces of the magnet bodies 35 in such a manner as to correspond with the thread crests 31 of the screw shaft 30, as shown in FIG. 9, N- and S-poles spirally arranged on the magnet bodies 35 lie alternately axially adjacent each other, with the result that the magnetic fluxes emanating from the N-poles immediately enter the S-poles.
As a result, formed between the magnetic nut 32 and the screw shaft 30, as shown in dash dot line in FIG. 9, is a magnetic circuit which extends from the N-pole of the magnet body 35 via two adjacent thread crests 31 of the screw shaft 30 to the S-pole of the magnet body 35: however, the number of magnetic fluxes passing through this magnetic circuit decreases, weakening the magnetic coupling between the magnetic nut 32 and the screw shaft 30.
Therefore, there is a disadvantage that the magnetic coupling between the magnetic nut 32 and the screw shaft 30 is so low that even if the magnetizing force of the individual N- and S-poles of the magnet bodies 35 are increased, the axial thrust force in the screw shaft 30 cannot be increased.
Further, as shown in FIG. 9, since the inner peripheral surfaces of the magnet bodies 35 are magnetized to spirally provide alternating N- and S-poles, there is a disadvantage that when the screw pitch is small, the magnetization itself of the inner peripheral surfaces of the magnet bodies 35 is very difficult or impossible to perform, which means that a magnetic screw having a small pitch cannot be produced.